LESSON 4: Triangles

- just like any polygon, triangle is one of the most popular geometric figures in Mathematics. It is the simplest three-sided polygon with various topics and practical applications on the field of mathematics and engineering. This is proven with the widespread topics and applications of triangles such as the Pythagorean Theorem, Trigonometric functions, laws of sine and cosine, bearing, and angles of elevation and depression.

- triangle is a polygon with three sides and three interior angles.

Classification of Triangles

1. According to sides

A. Equilateral triangle is a three sided polygon with three equal sides. 

a = b = c

B. Isosceles triangle is a three sided polygon with two equal sides.

C. Scalene triangle is a three sided polygon with no equal sides.

2. According to angles

A. Right triangle is a three sided polygon with one right angle. 

B. Oblique triangle is a triangle with no right angle.

I.  Equiangular triangle is a three sided polygon having three equal angles.

2. Acute triangle is a three sided polygon having three acute angles.

3. Obtuse triangle is a three sided polygon having one obtuse angle.

Special Lines in a Triangle

1. Median of a triangle is a segment connecting a vertex to the midpoint of the opposite side.

2. Angle bisector of a triangle is a segment from a vertex that bisects an angle and extends to the opposite side.

3. Altitude or height of a triangle is a segment from a vertex perpendicular to the opposite side.